Liquid crystal displays (LCDs) are increasingly replacing cathode ray tubes as displays in computers and in other video screen applications. This is particularly true for devices where minimizing size and weight are important design considerations. For example, LCDs are used almost exclusively in portable computers and laptops. Two types of standard architectures have emerged for LCDs: passive and active matrix. Passive LCDs use a pair of electrode plates with a liquid crystal layer sandwiched between them. Each plate in a passive LCD has a set of lines that runs orthogonal to the lines in the opposing plate. Addressing individual pixels in a passive LCD amount to placing a signal of the pair of lines whose intersection defines the pixel location. AMLCDs, while a little more expensive than passive LCDs, are preferred for high performance applications because of their better viewing angle, and higher contrast and resolution than passive LCDs. Today, several trends are currently speeding the acceptance of both types of liquid crystal displays (i.e. active matrix and passive) in the marketplace--decreasing cost of manufacture, increasing pixel densities and resolution, increasingly faster screen refresh rates--to name a few.
The portable consumer electronic goods that are employing AMLCDs are typically battery powered. One appealing aspect to the consumer is the "mean time between battery recharge" for an item--the longer a good can operate on a fully charged set of batteries, the more desirable. Thus, it is desirable for component suppliers to such portable goods to have energy efficient components. The same is true for manufacturers of AMLCDs.
Typically, an AMLCD comprises a liquid crystal layer sandwiched between an active matrix and a common electrode plate. The active matrix itself comprises a collection of pixel structures. Each such pixel structure comprises a thin film transistor (TFT) and a pixel element, connected in such a fashion that the source and drain lines of the TFT are coupled to a data line and the pixel element respectively. A passivation layer is usually deposited over and covering both the TFT and the pixel element, usually consisting of indium-tin-oxide (ITO). One such AMLCD structure currently being used is a 6.3 million pixel, 13 inch diagonal display built by the Xerox Corporation. The structure of that particular AMLCD is described in commonly assigned U.S. Patent Application, application Ser. No. 08/235,011 (Attorney Docket Number D/94179), entitled "Thin-Film Structure with Dense Array of Binary Control Units for Presenting Images" (hereinafter the "Array" application), filed concurrently herewith and which is hereby incorporated by reference.
One way to reduce the amount of energy consumption of an AMLCD is by reducing the amount of voltage required to activate the liquid crystal layer. To activate the liquid crystal, a threshold voltage must be applied between the pixel element and the common electrode plate. Therefore, any change in design that allows the pixel element and the common electrode plate to reach the threshold voltage with a lesser amount of charge would save on energy consumption. One way to achieve threshold voltage with lesser amounts of charge is to ensure that no unnecessary voltage drop is occurring between the pixel element and the common electrode plate.
Therefore, it is desirable to design a AMLCD pixel structure that would avoid unnecessary voltage drops between the pixel element and the common electrode plate.
Thus, it is an object of the present invention to design a AMLCD pixel structure that decreases the amount of wasteful voltage drop that could otherwise be used to reach the threshold voltage needed to activate the liquid crystal.